The present invention relates to an injection-compression process for forming complex shaped items of elastomeric material, possibly including a metal reinforcing element. In particular, the present invention relates to a process for molding bearing seals.
Items of plastic material may be either injection or compression molded.
When injection molded, a mass of pressurized liquid resin is fed into the mold to fill an impression of the same shape as the item being produced. This method provides for producing even complex shaped items to a good degree of dimensional accuracy; which result, however, depends on maintaining the injection pressure inside the mold cavity, and hence on the injection nozzle remaining in communication with the impression, throughout the time taken for the resin injected into the mold cavity to set.
The above limitation poses no problem when working with thermoplastic resins which are set relatively rapidly by cooling. In the case of an elastomer, on the other hand, wherein cross-linking and setting are achieved relatively slowly by heating in the presence of sulfur compounds (curing), the heat transmitted by conduction between the mold and nozzle would also cure the excess material inside the nozzle, thus resulting in clogging, so that the nozzle must be cooled to keep the material inside it below the curing temperature.
The above method also preferably employs molds with multiple impressions, thus resulting in scrap and a high degree of thermal inertia, and, generally speaking, in the use of complex machines with fairly long operating cycles resulting in low output and high production cost.
In the case of rubber materials, therefore, compression molding is preferred, which enables a reduction in cycle time by the formation of separate feed and molding stations. In this case, in fact, the preformed green material, normally in the form of paste, is introduced at a loading station through which a number of single-impression molds are fed successively, are supplied with the necessary material for the molding operation, and, traveling along conveyor lines, are closed to a predetermined pressure and fed into a heating tunnel for curing. Though highly efficient and cheap to run, compression molding frequently presents technical limitations, due to failure of the green paste to fill highly complex shaped impressions.
For specific production requirements, combined injection-compression processes are known, such as that described in European Patent Application n. 0244783. This relates to the production of contact lenses using a machine with a prewidened mold cavity which is fed at low pressure with a quantity of resin material of a volume greater than that occupied at atmospheric pressure by the finished article, and, until the resin sets, is subjected to controlled pressure for gradually reducing its volume.
Even the above known process, however, presents drawbacks, due to the difficulty in regulating several basic parameters, such as the ratio between the volume injected and the volume of the impression, the time lapse between injection and compression, and the final pressure value. In the course of the above operations, in fact, an appropriately thick, solid outer film must be formed for preventing leakage of the liquid polymer--which would require cleaning of the machine, thus further increasing production time and cost--but which does not impair the finished article due to partial filling of the mold. What is more, though suitable for producing articles such as contact lenses, a prewidened mold cavity whose volume is subsequently reduced is hardly feasible for producing complex shaped articles.